How good/bad is the current Pedersoli Lightning?

[LonesomePigeon]

"I've got an AWA lightning in 44-40 that had light hits and failure to feed.  Turns out the sizing die I was using was not adjusted  to set the shoulder back enough to allow the cartridge to full chamber. By not chambering fully, the cartridge would not allow the bolt to travel far enough so the hammer would release the next cartridge.  This would also cause a light strike on the primer. I hope that made sense."

Forgive my ignorance but what is the "shoulder"?